High-bandwidth multimode optical fiber with reduced cladding effect

ABSTRACT

The present invention embraces an optical fiber that includes a central core having a graded-index profile with respect to an outer cladding. The optical fiber also includes an inner cladding, a depressed trench, and an outer cladding. The optical fiber achieves reduced bending losses and a high bandwidth with a reduced cladding effect for high-data-rate applications.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application is a continuation of commonly assigned U.S. applicationSer. No. 12/953,948 for a High-Bandwidth Multimode Optical Fiber withReduced Cladding Effect (filed Nov. 24, 2010, and published May 26,2011, as Publication No. 2011/0123161 A1), now U.S. Pat. No. 8,280,213.

U.S. patent application Ser. No. 12/953,948 claims the benefit ofcommonly assigned pending French application Ser. No. 09/58381 for a“Fibre Optique Multimode à Très Large Bande Passante avec une InterfaceCœur-Gaine Optimiséé” (filed Nov. 25, 2009, at the National Institute ofIndustrial Property (France)).

U.S. patent application Ser. No. 12/953,948 further claims the benefitof commonly assigned U.S. Patent Application No. 61/265,101 for a “FibreOptique Multimode à Très Large Bande Passante avec une InterfaceCœur-Gaine Optimiséé” (filed Nov. 30, 2009).

Each of the foregoing patent applications and patent applicationpublication is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of fiber optic transmission,and, more specifically, to a multimode optical fiber having reducedbending losses and a high bandwidth for high data rate applications.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one ormore coating layers) conventionally includes an optical fiber core,which transmits and/or amplifies an optical signal, and an opticalcladding, which confines the optical signal within the core.Accordingly, the refractive index of the core n_(c) is typically greaterthan the refractive index of the optical cladding n_(g) (i.e.,n_(c)>n_(g)).

For optical fibers, the refractive index profile is generally classifiedaccording to the graphical appearance of the function that associatesthe refractive index with the radius of the optical fiber.Conventionally, the distance r to the center of the optical fiber isshown on the x-axis, and the difference between the refractive index (atradius r) and the refractive index of the optical fiber's outer cladding(e.g., an outer optical cladding) is shown on the y-axis. The refractiveindex profile is referred to as a “step” profile, “trapezoidal” profile,“alpha” profile, or “triangular” profile for graphs having therespective shapes of a step, a trapezoid, an alpha, or a triangle. Thesecurves are generally representative of the optical fiber's theoreticalor set profile. Constraints in the manufacture of the optical fiber,however, may result in a slightly different actual profile.

Generally speaking, two main categories of optical fibers exist:multimode fibers and single-mode fibers. In a multimode optical fiber,for a given wavelength, several optical modes are propagatedsimultaneously along the optical fiber, whereas in a single-mode opticalfiber, the higher order modes are strongly attenuated. The typicaldiameter of a single-mode or multimode glass fiber is 125 microns. Thecore of a multimode optical fiber typically has a diameter of betweenabout 50 microns and 62.5 microns, whereas the core of a single-modeoptical fiber typically has a diameter of between about 6 microns and 9microns. Multimode systems are generally less expensive than single-modesystems, because multimode light sources, connectors, and maintenancecan be obtained at a lower cost.

Multimode optical fibers are commonly used for short-distanceapplications requiring a broad bandwidth, such as local networks or LAN(local area network). Multimode optical fibers have been the subject ofinternational standardization under the ITU-T G.651.1 recommendations,which, in particular, define criteria (e.g., bandwidth, numericalaperture, and core diameter) that relate to the requirements for opticalfiber compatibility. The ITU-T G.651.1 standard is hereby incorporatedby reference in its entirety.

In addition, the OM3 standard has been adopted to meet the demands ofhigh-bandwidth applications (i.e., a data rate higher than 1 GbE) overlong distances (i.e., distances greater than 300 meters). The OM3standard is hereby incorporated by reference in its entirety. With thedevelopment of high-bandwidth applications, the average core diameterfor multimode optical fibers has been reduced from 62.5 microns to 50microns.

Typically, an optical fiber should have the broadest possible bandwidthto perform well in a high-bandwidth application. For a given wavelength,the bandwidth of an optical fiber may be characterized in severaldifferent ways. Typically, a distinction is made between the so-called“overfilled launch” condition (OFL) bandwidth and the so-called“effective modal bandwidth” condition (EMB). The acquisition of the OFLbandwidth assumes the use of a light source exhibiting uniformexcitation over the entire radial surface of the optical fiber (e.g.,using a laser diode or light emitting diode (LED)).

Recently developed light sources used in high-bandwidth applications,such as VCSELs (Vertical Cavity Surface Emitting Lasers), exhibit aninhomogeneous excitation over the radial surface of the optical fiber.For this kind of light source, the OFL bandwidth is a less suitablemeasurement, and so it is preferable to use the effective modalbandwidth (EMB). The calculated effective bandwidth (EMBc) estimates theminimum EMB of a multimode optical fiber independent of the kind ofVCSEL used. The EMBc is obtained from a differential-mode-delay (DMD)measurement (e.g., as set forth in the FOTP-220 standard).

An exemplary method of measuring DMD and calculating the effective modalbandwidth can be found in the FOTP-220 standard, which is herebyincorporated by reference in its entirety. Further details on thistechnique are set forth in the following publications, each of which ishereby incorporated by reference: P. F. Kolesar and D. J. Mazzarese,“Understanding Multimode Bandwidth and Differential Mode DelayMeasurements and Their Applications,” Proceedings of the 51stInternational Wire and Cable Symposium, pp. 453-460; and D. Coleman andPhilip Bell, “Calculated EMB Enhances 10 GbE Performance Reliability forLaser-Optimized 50/125 μm Multimode Fiber,” Corning Cable SystemsWhitepaper.

FIG. 1 shows a schematic diagram of a DMD measurement according to thecriteria of the FOTP-220 standard as published in its TIA SCFO-6.6version of Nov. 22, 2002. FIG. 1 schematically represents a part of anoptical fiber (i.e., an optical core surrounded by an outer cladding). ADMD graph is obtained by successively injecting into the multimodeoptical fiber a light pulse having a given wavelength λ₀ with a radialoffset between each successive pulse. The delay of each pulse is thenmeasured after a given length of fiber L. Multiple identical lightpulses (i.e., light pulses having the same amplitude, wavelength, andfrequency) are injected with different radial offsets with respect tothe center of the multimode optical fiber's core. The injected lightpulse is depicted in FIG. 1 as a black dot on the optical core of theoptical fiber. In order to characterize an optical fiber with a50-micron diameter, the FOTP-220 standard recommends that at least 24individual measurements be carried out (i.e., at 24 different radialoffset values). From these measurements, it is possible to determine themodal dispersion and the calculated effective modal bandwidth (EMBc).

The TIA-492AAAC-A standard, which is hereby incorporated by reference inits entirety, specifies the performance requirements for50-micron-diameter multimode optical fibers used over long distances inEthernet high-bandwidth transmission network applications. The OM3standard requires, at a wavelength of 850 nanometers, an EMB of at least2,000 MHz·km. The OM3 standard assures error-free transmissions for adata rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4standard requires, at a wavelength of 850 nanometers, an EMB of at least4,700 MHz·km to obtain error-free transmissions for a data rate of 10Gb/s (10 GbE) up to a distance of 550 meters. The OM4 standard is herebyincorporated by reference in its entirety.

In a multimode optical fiber, the difference between the propagationtimes, or group delay times, of the several modes along the opticalfiber determine the bandwidth of the optical fiber. In particular, forthe same propagation medium (i.e., in a step-index multimode opticalfiber), the different modes have different group delay times. Thisdifference in group delay times results in a time lag between the pulsespropagating along different radial offsets of the optical fiber.

For example, as shown in the graph on the right side of FIG. 1, a timelag is observed between the individual pulses. This FIG. 1 graph depictseach individual pulse in accordance with its radial offset in microns(y-axis) and the time in nanoseconds (x-axis) the pulse took to passalong a given length of the optical fiber.

As depicted in FIG. 1, the location of the peaks along the x-axisvaries, which indicates a time lag (i.e., a delay) between theindividual pulses. This delay causes a broadening of the resulting lightpulse. Broadening of the light pulse increases the risk of the pulsebeing superimposed onto a trailing pulse and reduces the bandwidth(i.e., data rate) supported by the optical fiber. The bandwidth,therefore, is directly linked to the group delay time of the opticalmodes propagating in the multimode core of the optical fiber. Thus, toguarantee a broad bandwidth, it is desirable for the group delay timesof all the modes to be identical. Stated differently, the intermodaldispersion should be zero, or at least minimized, for a givenwavelength.

To reduce intermodal dispersion, the multimode optical fibers used intelecommunications generally have a core with a refractive index thatdecreases progressively from the center of the optical fiber to itsinterface with a cladding (i.e., an “alpha” core profile). Such anoptical fiber has been used for a number of years, and itscharacteristics have been described in “Multimode Theory of Graded-CoreFibers” by D. Gloge et al., Bell system Technical Journal 1973, pp.1563-1578, and summarized in “Comprehensive Theory of Dispersion inGraded-Index Optical Fibers” by G. Yabre, Journal of LightwaveTechnology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of theabove-referenced articles is hereby incorporated by reference in itsentirety.

A graded-index profile (i.e., an alpha-index profile) can be describedby a relationship between the refractive index value n and the distancer from the center of the optical fiber according to the followingequation:

$n = {n_{1}\sqrt{1 - {2{\Delta( \frac{r}{a} )}^{\alpha}}}}$

wherein,

α≧1, and α is a non-dimensional parameter that is indicative of theshape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\Delta = \frac{( {n_{1}^{2} - n_{0}^{2}} )}{2n_{1}^{2}}$

where n₀ is the minimum refractive index of the multimode core, whichmay correspond to the refractive index of the outer cladding (most oftenmade of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile)therefore has a core profile with a rotational symmetry such that alongany radial direction of the optical fiber the value of the refractiveindex decreases continuously from the center of the optical fiber's coreto its periphery. When a multimode light signal propagates in such agraded-index core, the different optical modes experience differingpropagation mediums (i.e., because of the varying refractive indices).This, in turn, affects the propagation speed of each optical modedifferently. Thus, by adjusting the value of the parameter α, it ispossible to obtain a group delay time that is virtually equal for all ofthe modes. Stated differently, the refractive index profile can bemodified to reduce or even eliminate intermodal dispersion.

In practice, however, a manufactured multimode optical fiber has agraded-index central core surrounded by an outer cladding of constantrefractive index. The core-cladding interface interrupts the core'salpha-index profile. Consequently, the multimode optical fiber's corenever corresponds to a theoretically perfect alpha profile (i.e., thealpha set profile). The outer cladding accelerates the higher-ordermodes with respect to the lower-order modes. This phenomenon is known asthe “cladding effect.” In DMD measurements, the responses acquired forthe highest radial positions (i.e., nearest the outer cladding) exhibitmultiple pulses, which results in a temporal spreading of the responsesignal. Therefore, bandwidth is diminished by this cladding effect.

Multimode optical fibers are commonly used for short-distanceapplications requiring a broad bandwidth, such as local area networks(LANs). In such applications, the optical fibers may be subjected toaccidental or otherwise unintended bending, which can modify the modepower distribution and the bandwidth of the optical fiber.

It is therefore desirable to achieve multimode optical fibers that areunaffected by bends having a radius of curvature of less than 10millimeters. One proposed solution involves adding a depressed trenchbetween the core and the cladding. Nevertheless, the position and thedepth of the trench can significantly affect the optical fiber'sbandwidth.

Japanese Patent Publication No. JP 2006/47719 A, which is herebyincorporated by reference in its entirety, discloses a graded indexoptical fiber having a depressed trench in its cladding. Nevertheless,the disclosed optical fiber exhibits higher bending losses than desiredand a relatively low bandwidth. Moreover, the disclosed optical fiber'scladding effect is not mentioned.

International Publication No. WO-A-2008/085851, which is herebyincorporated by reference in its entirety, discloses a graded-indexoptical fiber having a depressed trench in its cladding. Nevertheless,the disclosed optical fiber exhibits a relatively low bandwidth, and itscladding effect is not mentioned.

U.S. Patent Application Publication No. 2009/0154888, which is herebyincorporated by reference in its entirety, discloses a graded-indexfiber having a depressed trench in its cladding. Nevertheless, thedisclosed fiber exhibits a relatively low bandwidth, and its claddingeffect is not mentioned.

European Patent No. EP 0131729, which is hereby incorporated byreference in its entirety, discloses a graded-index optical fiber havinga depressed trench in its cladding. According to this document, toachieve a high bandwidth, the distance between the end of thegraded-index profile core and the beginning of the depressed trenchshould be between 0.5 micron and 2 microns. Nevertheless, the disclosedoptical fiber exhibits higher bending losses than desired. Moreover, thedisclosed optical fiber's cladding effect is not mentioned.

Therefore, a need exists for a graded-index multimode optical fiberhaving reduced bending losses and a high bandwidth with a reducedcladding effect for high-data-rate applications.

SUMMARY

In one aspect, the present invention embraces a multimode optical fiberthat includes a central core surrounded by an outer cladding. Thecentral core has a radius r₁ and an alpha-index profile with respect tothe outer cladding. An inner cladding is positioned between the centralcore and the outer cladding (e.g., immediately surrounding the centralcore). The inner cladding has (i) an outer radius r₂, (ii) a width w₂,and (iii) a refractive index difference Δn₂ with respect to the outercladding. A depressed trench is positioned between the inner claddingand the outer cladding (e.g., immediately surrounding the innercladding). The depressed trench has (i) an outer radius r_(t), (ii) awidth w_(t), and (iii) a refractive index difference Δn_(t) with respectto the outer cladding. Typically, the width w₂ of the inner cladding andthe refractive index difference Δn_(t) between the depressed trench andthe outer cladding satisfy the following inequality:

${\frac{5.6}{{1000 \times \Delta\; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{100 \times \Delta\; n_{t}} - 0.4} + 2.}$

In an exemplary embodiment, the alpha-index profile of the central corehas an alpha parameter α of between about 1.9 and 2.1 (e.g., betweenabout 2.04 and 2.1).

In yet another exemplary embodiment, the refractive index differencebetween the central core and the outer cladding has a maximum value Δn₁of between about 11×10⁻³ and 16×10⁻³.

In yet another exemplary embodiment, the refractive index difference Δn₂between the inner cladding and the outer cladding is between about−0.05×10⁻³ and 0.05×10⁻³.

In yet another exemplary embodiment, the refractive index differenceΔn_(t) between the depressed trench and the outer cladding is betweenabout −15×10⁻³ and −3×10⁻³.

In yet another exemplary embodiment, the width w_(t) of the depressedtrench is between about three microns and five microns.

In one particular embodiment, the depressed trench has a volume v_(t) ofbetween about 200%·μm² and 1,200%·μm² (e.g., between about 250%·μm² and750%·μm²).

In yet another exemplary embodiment, for two turns around a bend radiusof 15 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.1 dB (e.g., lessthan about 0.05 dB).

In yet another exemplary embodiment, for two turns around a bend radiusof 10 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.3 dB (e.g., lessthan about 0.1 dB).

In yet another exemplary embodiment, for two turns around a bend radiusof 7.5 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.4 dB (e.g., lessthan about 0.2 dB).

In yet another exemplary embodiment, for two turns around a bend radiusof 5 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 1 dB (e.g., lessthan about 0.3 dB).

In another particular embodiment, at a wavelength of 850 nanometers, themultimode optical fiber has a radial offset bandwidth at 24 microns(ROB24) of at least about 5,000 MHz·km, such as at least about 10,000MHz·km (e.g., at least about 15,000 MHz·km).

In yet another particular embodiment, at a wavelength of 850 nanometers,the multimode optical fiber has an OFL bandwidth of typically at leastabout 1,500 MHz·km (e.g., 3,500 MHz·km or more), more typically at leastabout 5,000 MHz·km (e.g., at least about 10,000 MHz·km).

In yet another exemplary embodiment, the multimode optical fiber has anumerical aperture of 0.2±0.015 (i.e., between 0.185 and 0.215).

In yet another exemplary embodiment, at a wavelength of 850 nanometers,the multimode optical fiber typically has an outer DMD value of lessthan about 0.33 ps/m, more typically less than about 0.25 ps/m (e.g.,less than about 0.14 ps/m).

In another aspect, the present invention embraces an optical-fibersystem that includes at least a portion of one of the present multimodeoptical fibers. The optical-fiber system typically has a data rate of atleast about 10 Gb/s over a distance of about 100 meters. In oneembodiment, the optical-fiber system has a data rate of at least about10 Gb/s over a distance of about 300 meters.

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the invention, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an exemplary DMD measurement method andgraph.

FIG. 2 graphically depicts the refractive index profile of an exemplaryoptical fiber according to the present invention.

FIG. 3 graphically depicts the bandwidth at a radial offset of 24microns (ROB24) as a function of the width of an inner cladding and adepressed trench's refractive index difference with respect to an outercladding.

FIG. 4 graphically depicts the bandwidth at a radial offset of 24microns (ROB24) as a function of the alpha parameter of the centralcore's alpha-index profile.

FIG. 5 graphically depicts the overfilled launch condition (OFL)bandwidth as a function of the alpha parameter of the central core'salpha-index profile.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber that achievesreduced bending losses and a high bandwidth with a reduced claddingeffect for high-data rate applications.

FIG. 2 depicts the refractive index profile of an exemplary opticalfiber in accordance with the present invention. The optical fiberincludes a central core having an outer radius r₁ and an alpharefractive index profile (i.e., an alpha-index profile) with respect toan outer cladding (e.g., an outer optical cladding) surrounding thecentral core. Typically, the core has a radius r₁ of about 25 microns.The refractive index difference between the central core and the outercladding typically has a maximum value Δn₁ of between about 11×10⁻³ and16×10⁻³ (e.g., between about 12×10⁻³ and 15×10⁻³). The central coretypically has an alpha profile with an alpha parameter of between about1.9 and 2.1. In a particular embodiment, the central core has an alphaprofile with an alpha parameter of between about 2.04 and 2.1, such asbetween about 2.06 and 2.08 (e.g., between about 2.07 and 2.08). Inanother particular embodiment, the central core has an alpha profilewith an alpha parameter of between about 2.05 and 2.08 (e.g., betweenabout 2.06 and 2.07).

For reasons of cost, the outer cladding is typically made of naturalsilica, but it may alternatively be made of doped silica.

The optical fiber includes an inner cladding positioned between thecentral core and the outer cladding. In one embodiment, the innercladding immediately surrounds the central core. The inner cladding hasan outer radius r₂, a width w₂, and a refractive index difference Δn₂with respect to the outer cladding. The refractive index difference Δn₂between the inner cladding and the outer cladding is typically betweenabout −0.05×10⁻³ and 0.05×10⁻³. More typically, and as illustrated inFIG. 2, the refractive index difference Δn₂ between the inner claddingand the outer cladding is approximately zero. The characteristics of theinner cladding facilitate the achievement of high bandwidths.

The fiber typically includes a depressed trench positioned between theinner cladding and the outer cladding. By way of example, the depressedtrench may immediately surround the inner cladding. The depressed trenchhas an outer radius r_(t), a width w_(t), and a refractive indexdifference Δn_(t) with respect to the outer cladding. The width w_(t) ofthe depressed trench is typically between about 3 and 5 microns (μm).

Typically, the term “depressed trench” is used to describe a radialportion of an optical fiber that has a refractive index that issubstantially less than the refractive index of the outer cladding. Inthis regard, the refractive index difference Δn_(t) between thedepressed trench and the outer cladding is typically between about−15×10⁻³ and −3×10⁻³, more typically between about −10×10⁻³ and −5×10⁻³.

Generally speaking, a refractive index difference with respect to theouter cladding can also be expressed as a percentage using the followingequation:

${\Delta\%(r)} = \frac{100 \times ( {{n(r)}^{2} - n_{cladding}^{2}} )}{2{n(r)}^{2}}$where n(r) is the comparative refractive index value as a function ofradial position (e.g., the refractive index n_(t) of the depressedtrench), and n_(cladding) is the refractive index value of the outercladding. Those of ordinary skill in the art will appreciate that thisequation can be used if the refractive index varies over a given sectionof the optical fiber (i.e., the refractive index value varies as afunction of radial position) or if the refractive index is constant overa given section.

Those of ordinary skill in the art will appreciate that the outercladding typically has a constant refractive index. That said, if theouter cladding has a non-constant refractive index, the refractive indexdifference (e.g., Δn₁, Δn₂, Δn_(t), Δ₁%, Δ₂%, or Δ_(t)%) between asection of the optical fiber and the outer cladding is measured usingthe refractive index of the inner most portion of the outer cladding(i.e., the portion of the outer cladding that is expected to moststrongly affect the propagation of optical signals within the opticalfiber).

A constant refractive index difference with respect to the outercladding can also be expressed as a percentage by the followingequation:

${\Delta\%} = \frac{100 \times ( {n^{2} - n_{cladding}^{2}} )}{2n^{2}}$where n is the comparative refractive index (e.g., the refractive indexn_(t) of the depressed trench), and n_(cladding) is the refractive indexof the outer cladding.

As used herein, the depressed trench's volume v_(t) is defined by thefollowing equation:

v_(t) = 2π × ∫_(r_(int))^(r_(t))Δ_(t)%(r) × r× 𝕕rin which Δ_(t)% (r) is the depressed trench's refractive indexdifference as a function of radial position with respect to the outercladding expressed in percentage, r_(t) is the outer radius of thedepressed trench, and r_(int) is the inner radius of the depressedtrench (e.g., the outer radius r₂ of the inner cladding). Those ofordinary skill in the art will appreciate that this equation can be usedif the depressed trench's refractive index difference varies (i.e., thetrench is non-rectangular) or if the refractive index difference isconstant (i.e., the depressed trench is rectangular).

If the refractive index difference between the depressed trench and theouter cladding is constant, the volume v_(t) of the depressed trench canalso be defined by the following equation:v _(t)=Δ_(t)%×π×(r _(t) ² −r _(int) ²)in which Δ_(t)% is the depressed trench's refractive index differencewith respect to the outer cladding expressed in percentage, r_(t) is theouter radius of the depressed trench, and r_(int) is the inner radius ofthe depressed trench (e.g., the outer radius r₂ of the inner cladding).

The volume v_(t) of the depressed trench is typically between about200%·μm² and 1,200%·μm². More typically, the volume v_(t) of thedepressed trench is between about 250%·μm² and 750%·μm². Thecharacteristics of the depressed trench facilitate the achievement oflow bending losses.

The refractive index difference Δn_(t) between the depressed trench andthe outer cladding and the width w_(t) of the depressed trench should behigh enough to facilitate low bending losses. Moreover, the width w₂ ofthe inner cladding and the refractive index difference Δn_(t) betweenthe depressed trench and the outer cladding facilitate the achievementof high bandwidths with a reduced cladding effect.

In this regard, the width w₂ of the inner cladding and the refractiveindex difference Δn_(t) between the depressed trench and the outercladding typically satisfy the following inequality:

${\frac{5.6}{{1000 \times \Delta\; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{100 \times \Delta\; n_{t}} - 0.4} + 2.}$This relation between the width w₂ of the inner cladding and therefractive index difference Δn_(t) of the depressed trench makes itpossible to achieve both low bending losses and a high bandwidth.

For example, at a wavelength of 850 nanometers, optical fibers inaccordance with the present invention typically have: (i) for two turnswith a bend radius (e.g., a radius of curvature) of 15 millimeters,bending losses of less than about 0.1 dB (e.g., less than about 0.05dB), (ii) for two turns with a bend radius of 10 millimeters, bendinglosses of less than about 0.3 dB (e.g., less than about 0.1 dB), (iii)for two turns with a bend radius of 7.5 millimeters, bending losses ofless than about 0.4 dB (e.g., less than about 0.2 dB), and (iv) for twoturns with a bend radius of 5 millimeters, bending losses of less thanabout 1 dB (e.g., less than about 0.3 dB).

The present optical fibers have improved bandwidth in comparison withconventional fiber designs. In particular, at a wavelength of 850nanometers, the present optical fibers typically have an OFL bandwidthof greater than about 5,000 MHz·km (e.g., greater than about 8,000MHz·km), more typically higher than about 10,000 MHz·km (e.g., at leastabout 12,000 MHz·km). In an exemplary embodiment, at a wavelength of 850nanometers, the present optical fibers have an OFL bandwidth of at leastabout 15,000 MHz·km (e.g., about 18,000 MHz·km or more).

The foregoing notwithstanding, OFL bandwidth is not the only parameterthat can be used to evaluate an optical fiber's suitability forhigh-data-rate applications. In this regard, limiting the claddingeffect in the optical fiber facilitates improved fiber performance inhigh-data-rate applications.

The cladding effect of an optical fiber may be determined byestablishing the optical fiber's Radial Offset Bandwidth (ROB). The ROBis typically determined using DMD measurements, which are obtained byinjecting an input pulse having (i) a wavelength of 850 nanometers and(ii) a spatial width of 5 microns+/−0.5 micron. Typically, the inputpulse is obtained by coupling a light source (e.g., a semiconductor ortitanium-sapphire laser) to a single-mode optical fiber having itsoutlet face positioned 10 microns or less from the inlet face of themultimode optical fiber. The temporal profile of the output pulse (i.e.,the light pulse emitted from the outlet end of the multimode opticalfiber) can be measured for each radial offset. The ROB at a radialoffset X (in microns), denoted ROBX, is calculated by utilizing theinformation contained in the broadening and the deformation of thetemporal profile of the output pulse obtained for an injection at theradial offset X for a given wavelength λ₀ (e.g., 850 nanometers), whichcorresponds to a given frequency f. A transfer function H^(X)(f) may beobtained using a Fourier transform and a pulse deconvolutioncorresponding to each radial offset.

In this regard, S_(e)(f) represents the Fourier transform of the inputpulse measured according to the TIA-455-220-A 5.1 standard, which ishereby incorporated by reference in its entirety. Similarly, S_(S)(f,X)represents the Fourier transform of the output pulse corresponding tothe X offset launch measured according to the TIA-455-220-A 5.1standard. Those having ordinary skill in the art will recognize that theFourier transform of the outlet pulse is a function of both frequency fand radial offset X.

For each offset launch X, a transfer function H^(X)(f) can be defined asfollows:

${H^{X}(f)} = {\frac{S_{S}( {f,X} )}{S_{e}(f)}.}$

ROBX is the −3 dB bandwidth of the transfer function H^(x)(f)corresponding to the response of the optical fiber for an injection at aradial offset of X in the DMD measurements.

In practice, the bandwidth is calculated for an attenuation of −1.5 dBand then extrapolated for an attenuation of −3 dB, assuming a Gaussianresponse, and multiplied by a factor of √2 (as is also the case for thecalculation of the effective bandwidth):ROBX=√{square root over (2)}f _(x)and10·log₁₀(H ^((x))(f _(x)))=−1.5.

An exemplary method of measuring DMD and calculating the effective modalbandwidth can be found in the FOTP-220 standard, which, as noted, isincorporated by reference in its entirety.

The radial offset bandwidth at a radial distance of 24 microns from thecenter of the optical fiber's central core (i.e., the ROB24 parameter)provides a good characterization of the cladding effect. A high ROB24 isindicative of a reduced cladding effect.

Optical fibers in accordance with the present invention typicallyexhibit a reduced cladding effect. In particular, optical fibers inaccordance with the present invention typically have an ROB24, at awavelength of 850 nanometers, of at least about 5,000 MHz·km (e.g., atleast about 7,000 MHz·km, such as about 9,000 MHz·km or higher), moretypically about 10,000 MHz·km or higher (e.g., at least about 12,500MHz·km). In an exemplary embodiment, at a wavelength of 850 nanometers,the present optical fibers have an ROB24 of at least about 15,000 MHz·km(e.g., about 20,000 MHz·km or more).

FIG. 3 graphically depicts simulated ROB24 measurements of an exemplaryoptical fiber, at a wavelength of 850 nanometers, as function of theinner cladding's width w₂ and the refractive index difference Δn_(t)between the depressed trench and the outer cladding. The left-handy-axis represents the inner cladding's width w₂. The x-axis representsthe depressed trench's refractive index difference Δn_(t). The valuesfor ROB24 corresponding to a (w₂, Δn_(t)) pair are shown in shades ofgray. The darkest shade corresponds to an ROB24 of 2 GHz·km (i.e., 2,000MHz·km), and the lightest shaded value corresponds to an ROB24 of 20GHz·km (i.e., 20,000 MHz·km).

The highest ROB24 values are situated above curve a and below curve b.Curves a and b are defined by the following functions:

${{{curve}\mspace{14mu} a\text{:}\mspace{14mu} w_{2}} = {\frac{5.6}{{1000 \times \Delta\; n_{t}} - 2.1} + 2.03}};$and

${{curve}\mspace{14mu} b\text{:}\mspace{14mu} w_{2}} = {\frac{3}{{100 \times \Delta\; n_{t}} - 0.4} + 2.}$Optical fibers that possess a pair of values (w₂, Δn_(t)) that arebetween the curves a and b will typically exhibit an ROB24 that ishigher than 5,000 MHz·km (e.g., higher than 10,000 MHz·km). Thus, thepresent optical fibers exhibit minimized cladding effect and aresuitable for use in high data rate applications.

An optical fiber's cladding effect may also be evaluated usingdifferential-mode-delay measurements acquired with an outer mask. Forexample, the differential mode delay value on the outer mask (i.e., theouter DMD) can be obtained using the method of the FOTP-220 standard.For optical fibers with a core diameter of 50±3 microns (i.e., a coreradius r₁ of between 23.5 microns and 26.5 microns) the outer DMD ismeasured using an outer mask of 0-23 microns. In other words, adifferential mode delay value on the outer mask of 0-23 microns ismeasured using the DMD method over the radial offset range from thecenter of the central core (i.e., 0 microns) to 23 microns. Thus,signals coming from a radial offset of greater than 23 microns areignored (e.g., between 23 and 25 microns for a core having a radius of25 microns). Those of ordinary skill in the art will recognize that thedimensions of an outer mask may be modified for optical fibers havinglarger or smaller core diameters. For example, a mask with largerdimensions (e.g., a larger inner and outer radius) might be used withrespect to a multimode optical fiber having a 62.5-micron diameter core.Similarly, a mask with smaller dimensions (e.g., a smaller inner andouter radius) might be used with respect to a multimode optical fiberhaving a core that is less than 50 microns.

The outer DMD originates from a plot for DMD measured over 750 meters offiber. The light source used is typically a pulsed Ti:Sapphire laseremitting at 850 nanometers. The source emits pulses of less than 40picoseconds at quarter height, and the RMS (Root Mean Square) spectralwidth is less than 0.1 nanometer.

An exemplary optical fiber according to the present invention exhibitsimproved outer DMD delay. In particular, at a wavelength of 850nanometers, the exemplary optical fiber typically exhibits an outer DMDdelay value of less than about 0.33 ps/m (e.g., less than 0.25 ps/m,such as 0.14 ps/m or less).

The advantages of the present invention will be more evident bycontrasting comparative optical fibers with an exemplary optical fiberaccording to the present invention. Table 1 (below) showsfiber-profile-parameters of an exemplary optical fiber and twocomparative optical fibers. Examples 1 and 3 are comparative opticalfibers. Example 2 is an exemplary optical fiber according to the presentinvention.

TABLE 1 Examples a (μm) Δn₁ (×10⁻³) w₂ (μm) Δn_(t) (×10⁻³) w_(t) (μm) 125 14 1.6 −3.5 3 2 25 14 1.4 −5.5 3 3 25 14 0.8 −6.5 3

The optical fibers depicted in Table 1 have a similar refractive indexprofile; however, the optical fibers have different values for the innercladding's width w₂ and the depressed trench's refractive indexdifference Δn_(t). The second optical fiber (i.e., Example 2) has aninner cladding width w₂ and a refractive index difference Δn_(t) betweenthe depressed trench and the outer cladding which comply with theinequality:

${\frac{5.6}{{1000 \times \Delta\; n_{t}} - 2.1} + 2.03} \leq w_{2} \leq {\frac{3}{{100 \times \Delta\; n_{t}} - 0.4} + 2.}$Thus, the optical fiber in Example 2 can achieve low bending losses anda high bandwidth.

Further aspects of the present invention are illustrated in FIGS. 4-5.FIG. 4 graphically depicts the bandwidth at a radial offset of 24microns (ROB24) as a function of the alpha parameter of the centralcore's alpha-index profile for Table 1's optical fibers (i.e., Examples1, 2, and 3). Similarly, FIG. 5 graphically depicts the OFL bandwidth asa function of the alpha parameter of the central core's alpha-indexprofile for Table 1's optical fibers (i.e., Examples 1, 2, and 3). Boththe ROB24 and the OFL bandwidth were measured using a wavelength of 850nanometers.

FIG. 4 shows that the exemplary optical fiber according to the presentinvention (i.e., Example 2) has an ROB24 higher than 5,000 MHz·km, whilethe comparative optical fibers have an ROB24 below 5,000 MHz·km.Moreover, optical fibers in accordance with the present invention canexhibit an ROB24 higher than 10,000 MHz·km, or even higher than 15,000MHz·km (e.g., 20,000 MHz·km or higher).

FIG. 5 shows that the exemplary optical fiber according to the presentinvention (i.e., Example 2) can exhibit an OFL bandwidth that is higherthan that of comparative optical fibers. In particular, the opticalfiber in Example 2 demonstrates an OFL bandwidth higher than 15,000MHz·km.

Therefore, the present optical fibers typically exhibit a reducedcladding effect.

In one embodiment, the present optical fibers comply with ITU-TRecommendation G.651.1, which is hereby incorporated by reference in itsentirety. Such exemplary optical fibers have a core diameter of 50microns and a numerical aperture of 0.2±0.015 (i.e., between 0.185 and0.215).

In one particular embodiment, the present optical fibers are compliantwith the OM3 standard. Thus, the optical fibers have: (i) at awavelength of 850 nanometers, an effective modal bandwidth EMB higherthan 2,000 MHz·km, (ii) at a wavelength of 850 nanometers, an outer DMDbelow 0.3 ps/m, (iii) at a wavelength of 850 nanometers, an OFLbandwidth higher than 1,500 MHz·km, and (iv) a numerical aperture ofbetween 0.185 and 0.215.

In another particular embodiment, the present optical fibers arecompliant with the OM4 standard. Thus, the optical fibers have: (i) at awavelength of 850 nanometers, an effective modal bandwidth EMB higherthan 4700 MHz·km, (ii) at a wavelength of 850 nanometers, an outer DMDof less than 0.14 ps/m, (iii) at a wavelength of 850 nanometers, an OFLbandwidth higher than 3,500 MHz·km, and (iv) a numerical aperture ofbetween 0.185 and 0.215. As noted, the OM3 standard and the OM4 standardare incorporated by reference in their entirety.

In another aspect, the present invention embraces an optical-fibersystem (e.g., a multimode optical system) that includes at least aportion of an optical fiber as disclosed herein in accordance with theinvention. In particular, the optical system can exhibit a data rate ofat least 10 Gb/s over at least 100 meters (e.g., 300 meters).

The present optical fibers may facilitate the reduction in overalloptical-fiber diameter. As will be appreciated by those having ordinaryskill in the art, a reduced-diameter optical fiber is cost-effective,requiring less raw material. Moreover, a reduced-diameter optical fiberrequires less deployment space (e.g., within a buffer tube and/or fiberoptic cable), thereby facilitating increased fiber count and/or reducedcable size.

Those having ordinary skill in the art will recognize that an opticalfiber with a primary coating (and an optional secondary coating and/orink layer) typically has an outer diameter of between about 235 micronsand about 265 microns (μm). The component glass fiber itself (i.e., theglass core and surrounding cladding layers) typically has a diameter ofabout 125 microns, such that the total coating thickness is typicallybetween about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fibertypically has an outer diameter of about 125 microns. With respect tothe optical fiber's surrounding coating layers, the primary coatingtypically has an outer diameter of between about 175 microns and about195 microns (i.e., a primary coating thickness of between about 25microns and 35 microns), and the secondary coating typically has anouter diameter of between about 235 microns and about 265 microns (i.e.,a secondary coating thickness of between about 20 microns and 45microns). Optionally, the present optical fiber may include an outermostink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduceddiameter (e.g., an outermost diameter between about 150 microns and 230microns). In this alternative optical fiber configuration, the thicknessof the primary coating and/or secondary coating is reduced, while thediameter of the component glass fiber is maintained at about 125microns. (Those having ordinary skill in the art will appreciate that,unless otherwise specified, diameter measurements refer to outerdiameters.)

By way of illustration, in such exemplary embodiments the primarycoating layer may have an outer diameter of between about 135 micronsand about 175 microns (e.g., about 160 microns), typically less than 165microns (e.g., between about 135 microns and 150 microns), and usuallymore than 140 microns (e.g., between about 145 microns and 155 microns,such as about 150 microns).

Moreover, in such exemplary embodiments the secondary coating layer mayhave an outer diameter of between about 150 microns and about 230microns (e.g., more than about 165 microns, such as 190-210 microns orso), typically between about 180 microns and 200 microns. In otherwords, the total diameter of the optical fiber is reduced to less thanabout 230 microns (e.g., between about 195 microns and 205 microns, andespecially about 200 microns). By way of further illustration, anoptical fiber may employ a secondary coating of about 197 microns at atolerance of +/−5 microns (i.e., a secondary-coating outer diameter ofbetween 192 microns to 202 microns). Typically, the secondary coatingwill retain a thickness of at least about 10 microns (e.g., an opticalfiber having a reduced thickness secondary coating of between 15 micronsand 25 microns).

In another alternative embodiment, the outer diameter of the componentglass fiber may be reduced to less than 125 microns (e.g., between about60 microns and 120 microns), perhaps between about 70 microns and 115microns (e.g., about 80-110 microns). This may be achieved, forinstance, by reducing the thickness of one or more cladding layers. Ascompared with the prior alternative embodiment, (i) the total diameterof the optical fiber may be reduced (i.e., the thickness of the primaryand secondary coatings are maintained in accordance with the prioralternative embodiment) or (ii) the respective thicknesses of theprimary and/or secondary coatings may be increased relative to the prioralternative embodiment (e.g., such that the total diameter of theoptical fiber might be maintained).

By way of illustration, with respect to the former, a component glassfiber having a diameter of between about 90 and 100 microns might becombined with a primary coating layer having an outer diameter ofbetween about 110 microns and 150 microns (e.g., about 125 microns) anda secondary coating layer having an outer diameter of between about 130microns and 190 microns (e.g., about 155 microns). With respect to thelatter, a component glass fiber having a diameter of between about 90and 100 microns might be combined with a primary coating layer having anouter diameter of between about 120 microns and 140 microns (e.g., about130 microns) and a secondary coating layer having an outer diameter ofbetween about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make theresulting optical fiber more susceptible to microbending attenuation.That said, the advantages of further reducing optical-fiber diameter maybe worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coatinglayers (e.g., a primary coating and a secondary coating). At least oneof the coating layers—typically the secondary coating—may be coloredand/or possess other markings to help identify individual fibers.Alternatively, a tertiary ink layer may surround the primary andsecondary coatings.

The present optical fibers may be manufactured by drawing from finalpreforms.

A final preform may be manufactured by providing a primary preform withan outer overcladding layer (i.e., an overcladding process). The outerovercladding layer typically consists of doped or undoped, natural orsynthetic, silica glass. Several methods are available for providing theouter overcladding layer.

In a first exemplary method, the outer overcladding layer may beprovided by depositing and vitrifying natural or synthetic silicaparticles on the outer periphery of the primary preform under theinfluence of heat. Such a process is known, for example, from U.S. Pat.Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which ishereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded usinga silica sleeve tube, which may or may not be doped. This sleeve tubemay then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be appliedvia an Outside Vapor Deposition (OVD) method. Here, a soot layer isfirst deposited on the outer periphery of a primary preform, and thenthe soot layer is vitrified to form glass.

The primary preforms may be manufactured via outside vapor depositiontechniques, such as Outside Vapor Deposition (OVD) and Vapor AxialDeposition (VAD). Alternatively, the primary preforms may bemanufactured via inside deposition techniques in which glass layers aredeposited on the inner surface of a substrate tube of doped or undopedsilica glass, such as Modified Chemical Vapor Deposition (MCVD), FurnaceChemical Vapor Deposition (FCVD), and Plasma Chemical Vapor Deposition(PCVD).

By way of example, the primary preforms may be manufactured using a PCVDprocess, which can precisely control the central core's gradientrefractive index profile.

A depressed trench, for instance, may be deposited on the inner surfaceof a substrate tube as part of the chemical vapor deposition process.More typically, a depressed trench may be manufactured either (i) byusing a fluorine-doped substrate tube as the starting point of theinternal deposition process for deposition of the gradient refractiveindex central core or (ii) by sleeving a fluorine-doped silica tube overthe gradient refractive index central core, which itself may be producedusing an outside deposition process (e.g., OVD or VAD). Accordingly, acomponent glass fiber manufactured from the resulting preform may have adepressed trench located at the periphery of its central core.

As noted, a primary preform may be manufactured via an inside depositionprocess using a fluorine-doped substrate tube. The resulting tubecontaining the deposited layers may be sleeved by one or more additionalfluorine-doped silica tubes so as to increase the thickness of adepressed trench, or to create a depressed trench having a varyingrefractive index over its width. Although not required, one or moreadditional sleeve tubes (e.g., fluorine-doped substrate tubes) may becollapsed onto the primary preform before an overcladding step iscarried out. The process of sleeving and collapsing is sometimesreferred to as jacketing and may be repeated to build several glasslayers on the outside of the primary preform.

The present optical fibers may be deployed in various structures, suchas those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosedwithin a buffer tube. For instance, optical fiber may be deployed ineither a single-fiber loose buffer tube or a multi-fiber loose buffertube. With respect to the latter, multiple optical fibers may be bundledor stranded within a buffer tube or other structure. In this regard,within a multi-fiber loose buffer tube, fiber sub-bundles may beseparated with binders (e.g., each fiber sub-bundle is enveloped in abinder). Moreover, fan-out tubing may be installed at the termination ofsuch loose buffer tubes to directly terminate loose buffered opticalfibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermostoptical fiber coating (i.e., tight buffered fiber) or otherwise surroundthe outermost optical-fiber coating or ink layer to provide an exemplaryradial clearance of between about 50 and 100 microns (i.e., a semi-tightbuffered fiber).

With respect to the former tight buffered fiber, the buffering may beformed by coating the optical fiber with a curable composition (e.g., aUV-curable material) or a thermoplastic material. The outer diameter oftight buffer tubes, regardless of whether the buffer tube is formed froma curable or non-curable material, is typically less than about 1,000microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may beincluded between the optical fiber and the buffer tube (e.g., to providea gliding layer).

As will be known by those having ordinary skill in the art, an exemplarybuffer tube enclosing optical fibers as disclosed herein may be formedof polyolefins (e.g., polyethylene or polypropylene), includingfluorinated polyolefins, polyesters (e.g., polybutylene terephthalate),polyamides (e.g., nylon), as well as other polymeric materials andblends. In general, a buffer tube may be formed of one or more layers.The layers may be homogeneous or include mixtures or blends of variousmaterials within each layer.

In this context, the buffer tube may be extruded (e.g., an extrudedpolymeric material) or pultruded (e.g., a pultruded, fiber-reinforcedplastic). By way of example, the buffer tube may include a material toprovide high temperature and chemical resistance (e.g., an aromaticmaterial or polysulfone material).

Although buffer tubes typically have a circular cross section, buffertubes alternatively may have an irregular or non-circular shape (e.g.,an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply besurrounded by an outer protective sheath or encapsulated within a sealedmetal tube. In either structure, no intermediate buffer tube isnecessarily required.

Multiple optical fibers as disclosed herein may be sandwiched,encapsulated, and/or edge bonded to form an optical fiber ribbon.Optical fiber ribbons can be divisible into subunits (e.g., atwelve-fiber ribbon that is splittable into six-fiber subunits).Moreover, a plurality of such optical fiber ribbons may be aggregated toform a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or aribbon stack in which the uppermost and lowermost optical fiber ribbonshave fewer optical fibers than those toward the center of the stack.This construction may be useful to increase the density of opticalelements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmissionelements in buffer tubes or cables, subject to other constraints (e.g.,cable or mid-span attenuation). The optical elements themselves may bedesigned for increased packing density. For example, the optical fibermay possess modified properties, such as improved refractive-indexprofile, core or cladding dimensions, or primary-coating thicknessand/or modulus, to improve microbending and macrobendingcharacteristics.

By way of example, a rectangular ribbon stack may be formed with orwithout a central twist (i.e., a “primary twist”). Those having ordinaryskill in the art will appreciate that a ribbon stack is typicallymanufactured with rotational twist to allow the tube or cable to bendwithout placing excessive mechanical stress on the optical fibers duringwinding, installation, and use. In a structural variation, a twisted (oruntwisted) rectangular ribbon stack may be further formed into acoil-like configuration (e.g., a helix) or a wave-like configuration(e.g., a sinusoid). In other words, the ribbon stack may possess regular“secondary” deformations.

As will be known to those having ordinary skill in the art, such opticalfiber ribbons may be positioned within a buffer tube or othersurrounding structure, such as a buffer-tube-free cable. Subject tocertain restraints (e.g., attenuation), it is desirable to increase thedensity of elements such as optical fibers or optical fiber ribbonswithin buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be positioned externally adjacent to and strandedaround a central strength member. This stranding can be accomplished inone direction, helically, known as “S” or “Z” stranding, or ReverseOscillated Lay stranding, known as “S-Z” stranding. Stranding about thecentral strength member reduces optical fiber strain when cable strainoccurs during installation and use.

Those having ordinary skill in the art will understand the benefit ofminimizing fiber strain for both tensile cable strain and longitudinalcompressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur duringinstallation, the cable will become longer while the optical fibers canmigrate closer to the cable's neutral axis to reduce, if not eliminate,the strain being translated to the optical fibers. With respect tolongitudinal compressive strain, which may occur at low operatingtemperatures due to shrinkage of the cable components, the opticalfibers will migrate farther away from the cable's neutral axis toreduce, if not eliminate, the compressive strain being translated to theoptical fibers.

In a variation, two or more substantially concentric layers of buffertubes may be positioned around a central strength member. In a furthervariation, multiple stranding elements (e.g., multiple buffer tubesstranded around a strength member) may themselves be stranded aroundeach other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers(e.g., loose or ribbonized fibers) may be simply placed externallyadjacent to the central strength member (i.e., the buffer tubes are notintentionally stranded or arranged around the central strength member ina particular manner and run substantially parallel to the centralstrength member).

Alternatively still, the present optical fibers may be positioned withina central buffer tube (i.e., the central buffer tube cable has a centralbuffer tube rather than a central strength member). Such a centralbuffer tube cable may position strength members elsewhere. For instance,metallic or non-metallic (e.g., GRP) strength members may be positionedwithin the cable sheath itself, and/or one or more layers ofhigh-strength yarns (e.g., aramid or non-aramid yarns) may be positionedparallel to or wrapped (e.g., contrahelically) around the central buffertube (i.e., within the cable's interior space). As will be understood bythose having ordinary skill in the art, such strength yarns providetensile strength to fiber optic cables. Likewise, strength members canbe included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers),which may reduce unwanted attenuation in fiber optic cables (e.g.,rectangular, flat ribbon cables or round, loose tube cables) that aresubjected to relatively tight bends (i.e., a low bend radius). Moreover,the presence of a lubricant on strength yarns (e.g., aramid strengthyarns) may facilitate removal of the cable jacketing by reducingunwanted bonding between the strength yarns and the surrounding cablejacket.

In other embodiments, the optical fibers may be placed within a slottedcore cable. In a slotted core cable, optical fibers, individually or asa fiber ribbon, may be placed within pre-shaped helical grooves (i.e.,channels) on the surface of a central strength member, thereby forming aslotted core unit. The slotted core unit may be enclosed by a buffertube. One or more of such slotted core units may be placed within aslotted core cable. For example, a plurality of slotted core units maybe helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitubecable design, whereby the optical fibers are stranded around themselveswithin a large multi-fiber loose buffer tube rather than around acentral strength member. In other words, the large multi-fiber loosebuffer tube is centrally positioned within the maxitube cable. Forexample, such maxitube cables may be deployed in optical ground wires(OPGW).

In another cabling embodiment, multiple buffer tubes may be strandedaround themselves without the presence of a central member. Thesestranded buffer tubes may be surrounded by a protective tube. Theprotective tube may serve as the outer casing of the fiber optic cableor may be further surrounded by an outer sheath. The protective tube maytightly or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additionalelements may be included within a cable core. For example, copper cablesor other active, transmission elements may be stranded or otherwisebundled within the cable sheath. Passive elements may also be placedwithin the cable core, such as between the interior walls of the buffertubes and the enclosed optical fibers. Alternatively and by way ofexample, passive elements may be placed outside the buffer tubes betweenthe respective exterior walls of the buffer tubes and the interior wallof the cable jacket, or within the interior space of a buffer-tube-freecable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or othermaterials containing water-swellable material and/or coated withwater-swellable materials (e.g., including super absorbent polymers(SAPs), such as SAP powder) may be employed to provide water blockingand/or to couple the optical fibers to the surrounding buffer tubeand/or cable jacketing (e.g., via adhesion, friction, and/orcompression). Exemplary water-swellable elements are disclosed incommonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which ishereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive,such as a silicone acrylate cross-linked by exposure to actinicradiation) may be provided on one or more passive elements (e.g.,water-swellable material) to bond the elements to the buffer tube. Anadhesive material may also be used to bond the water-swellable elementto optical fibers within the buffer tube. Exemplary arrangements of suchelements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 fora Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which ishereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain athixotropic composition (e.g., grease or grease-like gels) between theoptical fibers and the interior walls of the buffer tubes. For example,filling the free space inside a buffer tube with water-blocking,petroleum-based filling grease helps to block the ingress of water.Further, the thixotropic filling grease mechanically (i.e., viscously)couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, therebyhindering connection and splicing operations. Thus, the present opticalfibers may be deployed in dry cable structures (i.e., grease-free buffertubes).

Exemplary buffer tube structures that are free from thixotropic fillinggreases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for aCoupling Composition for Optical Fiber Cables (Parris et al.), which ishereby incorporated by reference in its entirety. Such buffer tubesemploy coupling compositions formed from a blend of high-molecularweight elastomeric polymers (e.g., about 35 weight percent or less) andoils (e.g., about 65 weight percent or more) that flow at lowtemperatures. Unlike thixotropic filling greases, the couplingcomposition (e.g., employed as a cohesive gel or foam) is typically dryand, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cableenclosing optical fibers as disclosed herein may have a sheath formedfrom various materials in various designs. Cable sheathing may be formedfrom polymeric materials such as, for example, polyethylene,polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon),polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylenepropylene, polyvinyl fluoride, or polyvinylidene difluoride), andethylene vinyl acetate. The sheath and/or buffer tube materials may alsocontain other additives, such as nucleating agents, flame-retardants,smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectricmaterial (e.g., non-conducting polymers), with or without supplementalstructural components that may be used to improve the protection (e.g.,from rodents) and strength provided by the cable sheath. For example,one or more layers of metallic (e.g., steel) tape, along with one ormore dielectric jackets, may form the cable sheathing. Metallic orfiberglass reinforcing rods (e.g., GRP) may also be incorporated intothe sheath. In addition, aramid, fiberglass, or polyester yarns may beemployed under the various sheath materials (e.g., between the cablesheath and the cable core), and/or ripcords may be positioned, forexample, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have acircular cross section, but cable sheaths alternatively may have anirregular or non-circular shape (e.g., an oval, trapezoidal, or flatcross-section).

By way of example, the present optical fiber may be incorporated intosingle-fiber drop cables, such as those employed for Multiple DwellingUnit (MDU) applications. In such deployments, the cable jacketing mustexhibit crush resistance, abrasion resistance, puncture resistance,thermal stability, and fire resistance as required by building codes. Anexemplary material for such cable jackets is thermally stable,flame-retardant polyurethane (PUR), which mechanically protects theoptical fibers yet is sufficiently flexible to facilitate easy MDUinstallations. Alternatively, a flame-retardant polyolefin or polyvinylchloride sheath may be used.

In general, and as will be known to those having ordinary skill in theart, a strength member is typically in the form of a rod orbraided/helically wound wires or fibers, though other configurationswill be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may bevariously deployed, including as drop cables, distribution cables,feeder cables, trunk cables, and stub cables, each of which may havevarying operational requirements (e.g., temperature range, crushresistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts,plenums, or risers. By way of example, an optical fiber cable may beinstalled in an existing duct or microduct by pulling or blowing (e.g.,using compressed air). An exemplary cable installation method isdisclosed in commonly assigned U.S. Pat. No. 7,574,095 for aCommunication Cable Assembly and Installation Method, (Lock et al.), andU.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized CommunicationCable Assembly and Installation Method, (Griffioen et al.), each ofwhich is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose orribbonized fibers) may be stranded (e.g., around a central strengthmember). In such configurations, an optical fiber cable's protectiveouter sheath may have a textured outer surface that periodically varieslengthwise along the cable in a manner that replicates the strandedshape of the underlying buffer tubes. The textured profile of theprotective outer sheath can improve the blowing performance of theoptical fiber cable. The textured surface reduces the contact surfacebetween the cable and the duct or microduct and increases the frictionbetween the blowing medium (e.g., air) and the cable. The protectiveouter sheath may be made of a low coefficient-of-friction material,which can facilitate blown installation. Moreover, the protective outersheath can be provided with a lubricant to further facilitate blowninstallation.

In general, to achieve satisfactory long-distance blowing performance(e.g., between about 3,000 to 5,000 feet or more), the outer cablediameter of an optical fiber cable should be no more than about 70 to 80percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an airblown fiber system. In an air blown fiber system, a network of unfilledcables or microducts is installed prior to the installation of opticalfibers. Optical fibers may subsequently be blown into the installedcables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the groundor, as an aerial cable, suspended from a pole or pylon. An aerial cablemay be self-supporting, or secured or lashed to a support (e.g.,messenger wire or another cable). Exemplary aerial fiber optic cablesinclude overhead ground wires (OPGW), all-dielectric self-supportingcables (ADSS), all dielectric lash cables (AD-Lash), and figure-eightcables, each of which is well understood by those having ordinary skillin the art. Figure-eight cables and other designs can be directly buriedor installed into ducts, and may optionally include a toning element,such as a metallic wire, so that they can be found with a metaldetector.

In addition, although the optical fibers may be further protected by anouter cable sheath, the optical fiber itself may be further reinforcedso that the optical fiber may be included within a breakout cable, whichallows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmissionsystem, connections are required at various points in the network.Optical fiber connections are typically made by fusion splicing,mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber endseither in the field (e.g., at the network location) or in a factoryprior to installation into the network. The ends of the connectors aremated in the field in order to connect the optical fibers together orconnect the optical fibers to the passive or active components. Forexample, certain optical fiber cable assemblies (e.g., furcationassemblies) can separate and convey individual optical fibers from amultiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplementalequipment, which itself may employ the present optical fiber aspreviously disclosed. For instance, an amplifier may be included toimprove optical signals. Dispersion compensating modules may beinstalled to reduce the effects of chromatic dispersion and polarizationmode dispersion. Splice boxes, pedestals, and distribution frames, whichmay be protected by an enclosure, may likewise be included. Additionalelements include, for example, remote terminal switches, optical networkunits, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use ina communication system (e.g., networking or telecommunications). Acommunication system may include fiber optic cable architecture such asfiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure(FTTE), fiber-to-the-curb (FITC), fiber-to-the-building (FTTB), andfiber-to-the-home (FTTH), as well as long-haul or metro architecture.Moreover, an optical module or a storage box that includes a housing mayreceive a wound portion of the optical fiber disclosed herein. By way ofexample, the optical fiber may be wound with a bending radius of lessthan about 15 millimeters (e.g., 10 millimeters or less, such as about 5millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications,including, without limitation, fiber optic sensors or illuminationapplications (e.g., lighting).

The present optical fibers may include Fiber Bragg Grating (FBG). Aswill be known by those having ordinary skill in the art, FBG is aperiodic or aperiodic variation in the refractive index of an opticalfiber core and/or cladding. This variation in the refractive indexresults in a range of wavelengths (e.g., a narrow range) being reflectedrather than transmitted, with maximum reflectivity occurring at theBragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber byexposing the optical fiber to an intense source of ultraviolet light(e.g., a UV laser). In this respect, UV photons may have enough energyto break molecular bonds within an optical fiber, which alters thestructure of the optical fiber, thereby increasing the optical fiber'srefractive index. Moreover, dopants (e.g., boron or germanium) and/orhydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation ofFBG, the coating may be removed. Alternatively, coatings that aretransparent at the particular UV wavelengths (e.g., the UV wavelengthsemitted by a UV laser to write FBG) may be employed to render coatingremoval unnecessary. In addition, silicone, polyimide, acrylate, or PFCBcoatings, for instance, may be employed for high-temperatureapplications.

A particular FBG pattern may be created by employing (i) a photomaskplaced between the UV light source and the optical fiber, (ii)interference between multiple UV light beams, which interfere with eachother in accordance with the desired FBG pattern (e.g., a uniform,chirped, or titled pattern), or (iii) a narrow UV light beam forcreating individual variations. The FBG structure may have, for example,a uniform positive-only index change, a Gaussian-apodized index change,a raised-cosine-apodized index change, or a discrete phase shift indexchange. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensingapplications (e.g., for detecting vibration, temperature, pressure,moisture, or movement). In this respect, changes in the optical fiber(e.g., a change in temperature) result in a shift in the Braggwavelength, which is measured by a sensor. FBG may be used to identify aparticular optical fiber (e.g., if the optical fiber is broken intopieces).

Fiber Bragg Grating may also be used in various active or passivecommunication components (e.g., wavelength-selective filters,multiplexers, demultiplexers, Mach-Zehnder interferometers, distributedBragg reflector lasers, pump/laser stabilizers, and supervisorychannels).

To supplement the present disclosure, this application incorporatesentirely by reference the following commonly assigned patents, patentapplication publications, and patent applications: U.S. Pat. No.4,838,643 for a Single Mode Bend Insensitive Fiber for Use in FiberOptic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 fora Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No.7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S.Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (deMontmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic DispersionCompensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,555,186 for anOptical Fiber (Flammer et al.); U.S. Patent Application Publication No.US2009/0252469 A1 for a Dispersion-Shifted Optical Fiber (Sillard etal.); U.S. patent application Ser. No. 12/098,804 for a TransmissionOptical Fiber Having Large Effective Area (Sillard et al.), filed Apr.7, 2008; International Patent Application Publication No. WO 2009/062131A1 for a Microbend-Resistant Optical Fiber, (Overton); U.S. PatentApplication Publication No. US2009/0175583 A1 for a Microbend-ResistantOptical Fiber, (Overton); U.S. Patent Application Publication No.US2009/0279835 A1 for a Single-Mode Optical Fiber Having Reduced BendingLosses, filed May 6, 2009, (de Montmorillon et al.); U.S. PatentApplication Publication No. US2009/0279836 A1 for a Bend-InsensitiveSingle-Mode Optical Fiber, filed May 6, 2009, (de Montmorillon et al.);U.S. Patent Application Publication No. US2010/0021170 A1 for aWavelength Multiplexed Optical System with Multimode Optical Fibers,filed Jun. 23, 2009, (Lumineau et al.); U.S. Patent ApplicationPublication No. US2010/0028020 A1 for a Multimode Optical Fibers, filedJul. 7, 2009, (Gholami et al.); U.S. Patent Application Publication No.US2010/0119202 A1 for a Reduced-Diameter Optical Fiber, filed Nov. 6,2009, (Overton); U.S. Patent Application Publication No. US2010/0142969A1 for a Multimode Optical System, filed Nov. 6, 2009, (Gholami et al.);U.S. Patent Application Publication No. US2010/0118388 A1 for anAmplifying Optical Fiber and Method of Manufacturing, filed Nov. 12,2009, (Pastouret et al.); U.S. Patent Application Publication No.US2010/0135627 A1 for an Amplifying Optical Fiber and Production Method,filed Dec. 2, 2009, (Pastouret et al.); U.S. Patent ApplicationPublication No. US2010/0142033 for an Ionizing Radiation-ResistantOptical Fiber Amplifier, filed Dec. 8, 2009, (Regnier et al.); U.S.Patent Application Publication No. US2010/0150505 A1 for a BufferedOptical Fiber, filed Dec. 11, 2009, (Testu et al.); U.S. PatentApplication Publication No. US2010/0171945 for a Method of Classifying aGraded-Index Multimode Optical Fiber, filed Jan. 7, 2010, (Gholami etal.); U.S. Patent Application Publication No. US2010/0189397 A1 for aSingle-Mode Optical Fiber, filed Jan. 22, 2010, (Richard et al.); U.S.Patent Application Publication No. US2010/0189399 A1 for a Single-ModeOptical Fiber Having an Enlarged Effective Area, filed Jan. 27, 2010,(Sillard et al.); U.S. Patent Application Publication No. US2010/0189400A1 for a Single-Mode Optical Fiber, filed Jan. 27, 2010, (Sillard etal.); U.S. Patent Application Publication No. US2010/0214649 A1 for anOptical Fiber Amplifier Having Nanostructures, filed Feb. 19, 2010,(Burow et al.); U.S. Patent Application Publication No. US2010/0254653A1 for a Multimode Fiber, filed Apr. 22, 2010, (Molin et al.); U.S.patent application Ser. No. 12/794,229 for a Large Bandwidth MultimodeOptical Fiber Having a Reduced Cladding Effect, filed Jun. 4, 2010,(Molin et al.); U.S. patent application Ser. No. 12/878,449 for aMultimode Optical Fiber Having Improved Bending Losses, filed Sep. 9,2010, (Molin et al.); U.S. patent application Ser. No. 12/884,834 for aMultimode Optical Fiber, filed Sep. 17, 2010, (Molin et al.); U.S.patent application Ser. No. 12/887,813 for an Optical Fiber forSum-Frequency Generation, filed Sep. 22, 2010, (Richard et al.); andU.S. patent application Ser. No. 12/954,036 for a High-Bandwidth,Dual-Trench-Assisted Multimode Optical Fiber, filed Nov. 24, 2010,(Molin et al.).

To supplement the present disclosure, this application furtherincorporates entirely by reference the following commonly assignedpatents, patent application publications, and patent applications: U.S.Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubesfor Optical Fiber Cables and Method for Making the Same; U.S. Pat. No.5,717,805 for Stress Concentrations in an Optical Fiber Ribbon toFacilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical FiberCables and Method for Making the Same; U.S. Pat. No. 5,911,023 forPolyolefin Materials Suitable for Optical Fiber Cable Components; U.S.Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbonto Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No.6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No.6,066,397 for Polypropylene Filler Rods for Optical Fiber CommunicationsCables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon andMethod for Making the Same; U.S. Pat. No. 6,085,009 for Water BlockingGels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes andCables Made Therewith; U.S. Pat. No. 6,215,931 for FlexibleThermoplastic Polyolefin Elastomers for Buffering Transmission Elementsin a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method forAccessing Optical Fibers in the Midspan Region of an Optical FiberCable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbonand Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method forAccessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section;U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix MaterialHaving Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for anOptical Fiber Having Water Swellable Material for Identifying Groupingof Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for ManufacturingOptical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene FillerRods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491for an Optical Drop Cable for Aerial Installation; U.S. Pat. No.7,346,244 for a Coated Central Strength Member for Fiber Optic Cableswith Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skinfor Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube thatResults in Easy Access to and Low Attenuation of Fibers Disposed WithinBuffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-SpeedGel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446for an Optical Fiber Cable with Cushion Members Protecting Optical FiberRibbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus toReduce Variation of Excess Fiber Length in Buffer Tubes of Fiber OpticCables; U.S. Pat. No. 6,618,538 for a Method and Apparatus to ReduceVariation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables;U.S. Pat. No. 7,322,122 for a Method and Apparatus for Curing a FiberHaving at Least Two Fiber Coating Curing Stages; U.S. Pat. No. 6,912,347for an Optimized Fiber Optic Cable Suitable for Microduct BlownInstallation; U.S. Pat. No. 6,941,049 for a Fiber Optic Cable Having NoRigid Strength Members and a Reduced Coefficient of Thermal Expansion;U.S. Pat. No. 7,162,128 for Use of Buffer Tube Coupling Coil to PreventFiber Retraction; U.S. Pat. No. 7,515,795 for a Water-Swellable Tape,Adhesive-Backed for Coupling When Used Inside a Buffer Tube (Overton etal.); U.S. Patent Application Publication No. 2008/0292262 for aGrease-Free Buffer Optical Fiber Buffer Tube Construction Utilizing aWater-Swellable, Texturized Yarn (Overton et al.); European PatentApplication Publication No. 1,921,478 A1, for a TelecommunicationOptical Fiber Cable (Tatat et al.); U.S. Pat. No. 7,702,204 for a Methodfor Manufacturing an Optical Fiber Preform (Gonnet et al.); U.S. Pat.No. 7,570,852 for an Optical Fiber Cable Suited for Blown Installationor Pushing Installation in Microducts of Small Diameter (Nothofer etal.); U.S. Pat. No. 7,526,177 for a Fluorine-Doped Optical Fiber(Matthijsse et al.); U.S. Pat. No. 7,646,954 for an Optical FiberTelecommunications Cable (Tatat); U.S. Pat. No. 7,599,589 for a Gel-FreeBuffer Tube with Adhesively Coupled Optical Element (Overton et al.);U.S. Pat. No. 7,567,739 for a Fiber Optic Cable Having a Water-SwellableElement (Overton); U.S. Pat. No. 7,817,891 for a Method for AccessingOptical Fibers within a Telecommunication Cable (Lavenne et al.); U.S.Pat. No. 7,639,915 for an Optical Fiber Cable Having a DeformableCoupling Element (Parris et al.); U.S. Pat. No. 7,646,952 for an OpticalFiber Cable Having Raised Coupling Supports (Parris); U.S. Pat. No.7,724,998 for a Coupling Composition for Optical Fiber Cables (Parris etal.); U.S. Patent Application Publication No. US2009/0214167 A1 for aBuffer Tube with Hollow Channels, (Lookadoo et al.); U.S. PatentApplication Publication No. US2009/0297107 A1 for an Optical FiberTelecommunication Cable, filed May 15, 2009, (Tatat); U.S. PatentApplication Publication No. US2009/0279833 A1 for a Buffer Tube withAdhesively Coupled Optical Fibers and/or Water-Swellable Element, filedJul. 21, 2009, (Overton et al.); U.S. Patent Application Publication No.US2010/0092135 A1 for an Optical Fiber Cable Assembly, filed Sep. 10,2009, (Barker et al.); U.S. Patent Application Publication No.US2010/0067857 A1 for a High-Fiber-Density Optical Fiber Cable, filedSep. 10, 2009, (Louie et al.); U.S. Patent Application Publication No.US2010/0067855 A1 for a Buffer Tubes for Mid-Span Storage, filed Sep.11, 2009, (Barker); U.S. Patent Application Publication No.US2010/0135623 A1 for Single-Fiber Drop Cables for MDU Deployments,filed Nov. 9, 2009, (Overton); U.S. Patent Application Publication No.US2010/0092140 A1 for an Optical-Fiber Loose Tube Cables, filed Nov. 9,2009, (Overton); U.S. Patent Application Publication No. US2010/0135624A1 for a Reduced-Size Flat Drop Cable, filed Nov. 9, 2009, (Overton etal.); U.S. Patent Application Publication No. US2010/0092138 A1 for ADSSCables with High-Performance Optical Fiber, filed Nov. 9, 2009,(Overton); U.S. Patent Application Publication No. US2010/0135625 A1 forReduced-Diameter Ribbon Cables with High-Performance Optical Fiber,filed Nov. 10, 2009, (Overton); U.S. Patent Application Publication No.US2010/0092139 A1 for a Reduced-Diameter, Easy-Access Loose Tube Cable,filed Nov. 10, 2009, (Overton); U.S. Patent Application Publication No.US2010/0154479 A1 for a Method and Device for Manufacturing an OpticalPreform, filed Dec. 19, 2009, (Milicevic et al.); U.S. PatentApplication Publication No. US 2010/0166375 for a PerforatedWater-Blocking Element, filed Dec. 29, 2009, (Parris); U.S. PatentApplication Publication No. US2010/0183821 A1 for a UVLED Apparatus forCuring Glass-Fiber Coatings, filed Dec. 30, 2009, (Hartsuiker et al.);U.S. Patent Application Publication No. US2010/0202741 A1 for aCentral-Tube Cable with High-Conductivity Conductors Encapsulated withHigh-Dielectric-Strength Insulation, filed Feb. 4, 2010, (Ryan et al.);U.S. Patent Application Publication No. US2010/0215328 A1 for a CableHaving Lubricated, Extractable Elements, filed Feb. 23, 2010, (Tatat etal.); U.S. patent application Ser. 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In the specification and/or figures, typical embodiments of theinvention have been disclosed. The present invention is not limited tosuch exemplary embodiments. The use of the term “and/or” includes anyand all combinations of one or more of the associated listed items. Thefigures are schematic representations and so are not necessarily drawnto scale. Unless otherwise noted, specific terms have been used in ageneric and descriptive sense and not for purposes of limitation.

1. A multimode optical fiber, comprising: a central core surrounded byan outer cladding, said central core having a radius r₁ and agraded-index profile with respect to said outer cladding; an innercladding positioned between said central core and said outer cladding,said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and(iii) a refractive index difference Δn₂ with respect to said outercladding; and a depressed trench positioned between said inner claddingand said outer cladding, said depressed trench having a refractive indexdifference Δn_(t) with respect to said outer cladding; wherein saidinner cladding's width w₂ and said depressed trench's refractive indexdifference Δn_(t) satisfy the following inequality:${\frac{5.6}{{1000 \times \Delta\; n_{t}} - 2.1} + 2.03} \leq {w_{2}.}$2. A multimode optical fiber according to claim 1, wherein said centralcore's graded-index profile is an alpha-index profile having an alphaparameter α of between about 1.9 and 2.1.
 3. A multimode optical fiberaccording to claim 1, wherein, with respect to said outer cladding, saidcentral core has a maximum refractive index difference Δn_(t) of betweenabout 11×10⁻³ and 16×10⁻³.
 4. A multimode optical fiber according toclaim 1, wherein said inner cladding immediately surrounds said centralcore and said depressed trench immediately surrounds said innercladding.
 5. A multimode optical fiber according to claim 1, whereinsaid inner cladding's refractive index difference Δn₂ is between about−0.05×10⁻³ and 0.05×10⁻³.
 6. A multimode optical fiber according toclaim 1, wherein said depressed trench's refractive index differenceΔn_(t) is between about −15×10⁻³ and −3×10⁻³.
 7. A multimode opticalfiber according to claim 1, wherein said depressed trench has a volumev_(t) of between about 200%·μm² and 1,200%·μm².
 8. A multimode opticalfiber according to claim 1, wherein, for two turns around a bend radiusof 15 millimeters at a wavelength of 850 nanometers, the multimodeoptical fiber has bending losses of less than about 0.1 dB.
 9. Amultimode optical fiber according to claim 1, wherein, for two turnsaround a bend radius of 10 millimeters at a wavelength of 850nanometers, the multimode optical fiber has bending losses of less thanabout 0.3 dB.
 10. A multimode optical fiber according to claim 1,wherein, for two turns around a bend radius of 7.5 millimeters at awavelength of 850 nanometers, the multimode optical fiber has bendinglosses of less than about 0.2 dB.
 11. A multimode optical fiberaccording to claim 1, wherein, for two turns around a bend radius of 5millimeters at a wavelength of 850 nanometers, the multimode opticalfiber has bending losses of less than about 0.3 dB.
 12. A multimodeoptical fiber according to claim 1, wherein, at a wavelength of 850nanometers, the multimode optical fiber has a radial offset bandwidth at24 microns (ROB24) of at least about 5,000 MHz·km.
 13. A multimodeoptical fiber according to claim 1, wherein, at a wavelength of 850nanometers, the multimode optical fiber has a radial offset bandwidth at24 microns (ROB24) of at least about 10,000 MHz·km.
 14. A multimodeoptical fiber according to claim 1, wherein, at a wavelength of 850nanometers, the multimode optical fiber has a radial offset bandwidth at24 microns (ROB24) of at least about 15,000 MHz·km.
 15. A multimodeoptical fiber according to claim 1, wherein, at a wavelength of 850nanometers, the multimode optical fiber has an OFL bandwidth of at leastabout 1,500 MHz·km.
 16. A multimode optical fiber according to claim 1,wherein, at a wavelength of 850 nanometers, the multimode optical fiberhas an OFL bandwidth of at least about 3,500 MHz·km.
 17. A multimodeoptical fiber according to claim 1, wherein the multimode optical fiberhas a numerical aperture of 0.2±0.015.
 18. A multimode optical fiberaccording to claim 1, wherein, at a wavelength of 850 nanometers, themultimode optical fiber has an outer DMD value of less than about 0.33ps/m.
 19. A multimode optical fiber according to claim 1, wherein: saidcentral core's radius r₁ is 25 microns±1.5 microns; and at a wavelengthof 850 nanometers, the multimode optical fiber has an outer DMD value(0-23 microns) of less than about 0.14 ps/m.
 20. An optical-fiber systemcomprising at least a portion of the multimode optical fiber accordingto claim
 1. 21. An optical-fiber system according to claim 20, whereinthe optical-fiber system has a data rate of at least about 10 Gb/s overa distance of about 100 meters.
 22. An optical-fiber system according toclaim 20, wherein the optical-fiber system has a data rate of at leastabout 10 Gb/s over a distance of about 300 meters.